In telephone switching equipment, communication equipment, computer equipment, and in many other applications, the need for an uninterrupted source of DC power is critical. Rectified commercial AC power is typically used as the primary source of DC power for such equipment.
To avoid any interruption or outage in power service, it is common practice to employ a battery back-up for the primary DC source. Back-up battery systems typically include strings of batteries or cells connected in parallel with the primary DC source and the load. In the event of a drop in the load bus voltage below a predetermined threshold, the back-up battery supplants or supplements the primary source of DC power. Back-up battery systems are designed to replace the primary DC power source for a predetermined period of time within which resumption of primary power is expected to occur.
In conventional back-up battery systems, the nominal system load bus voltage has typically been dictated by battery characteristics. For example, in a telephone switching plant, back-up batteries commonly employed have a design cell voltage of 2.26 volts and are connected to provide negative voltage to a load. Twenty-four of these cells are typically combined in a string, resulting in a nominal load bus voltage of approximately xe2x88x9254 volts. A bank of strings supplies the necessary back-up DC power.
As the back-up batteries are placed across the load, the full xe2x88x9254 volts of system DC voltage are placed across the battery string. This design architecture of a typical back-up battery system presents a number of potential problems. Certain batteries, due to their electrochemical constitution, will draw more current than other batteries. All batteries, as they age, will experience increasing internal resistance and will draw more charging current from the main DC supply.
In recent years, a newer type of lead acid battery was introduced into the marketplace. The battery is sealed, and purportedly requires no maintenance. In this type of battery, oxygen and hydrogen produced during electrochemical reactions in the battery recombine to maintain an aqueous liquid electrolyte at a constant level within the cell. As a result, these batteries have only a small amount of liquid electrolyte. These batteries have become known as xe2x80x9cvalve regulatedxe2x80x9d, or xe2x80x9crecombinantxe2x80x9d or xe2x80x9celectrolyte-starvedxe2x80x9d batteries. This type of lead acid battery (hereinafter termed xe2x80x9cvalve regulated lead acidxe2x80x9d or xe2x80x9cVRLAxe2x80x9d batteries) has often failed well before their design life, which is typically 10 years.
It has been observed that a battery, over time, may begin to take on greater current to maintain its charge. This increasing charging current will elevate the temperature of the battery. The chemical recombination of the oxygen and hydrogen gases also creates heat. As the internal battery temperature increases, the current demand increases disproportionately. For every 10 degrees Celsius of increase in the battery""s internal temperature, the current demand doubles. A battery in this condition will have one of two failure modes, the most damaging being xe2x80x9cthermal runaway.xe2x80x9d Thermal runaway may lead to an explosion of the battery, with likely destruction or severe damage to any nearby equipment. Alternatively, the battery may experience a xe2x80x9cmelt downxe2x80x9d and produce noxious gases which are also apt to damage or destroy neighboring equipment. Moreover, the rectified AC source provided in typical telephone switching plants has more than ample capacity to supply any one or more batteries demanding abnormal charging current, thus encouraging thermal runaway or meltdown failures.
With the advent of fiber optic signal distribution, communications switching equipment has been decentralized, introducing a need for DC power supplies in unattended satellite installations distributed throughout the territory served. In these unattended installations, the equipment is often closely packed, leading to hostile thermal operating conditions for the equipment and increased occurrences of thermally induced failures. In less severe conditions, the placement of the back-up batteries directly across the load is apt to result in dry-out (loss of electrolyte), positive grid corrosion, and other problems which may lead to premature battery failure and/or sub-normal power performance.
Back-up battery systems must be monitored to determine the health and capacity of the batteries. The need to perform battery tests is particularly troublesome in systems that require the supply of an uninterrupted source of DC power. Testing of the vital statistics of a battery affecting output capacity, predicted life, etc. is presently done by taking the battery strings off-line and testing them in one of two ways. The test procedure recommended by battery manufacturers as being the most reliable is to discharge the battery into a load while measuring the response of the battery. The ability of a battery or battery string to hold a predetermined current level for a predetermined time is a reliable measure of the health and capacity of the battery. However, such discharge tests in the field require experienced personnel and are difficult and costly. Further, conventional battery testing, requiring the batteries to be taken off-line, suffers a loss of standby battery protection for the telephone plant or other equipment being supplied while the tested batteries are off-line.
To avoid the cost and inconvenience of a discharge test, special field test equipment is commonly employed that tests for battery resistance, impedance, inductance, and other parameters and characteristics without discharging the battery. See U.S. Pat. No. 5,250,904. However, as noted, tests that do not involve discharging the battery are apt to be less reliable.
U.S. Pat. No. 5,160,851 discloses a back-up battery system for telephone central office switching equipment. The back-up battery system includes one or more rechargeable batteries having cells floated at a given float voltage. The cells are of a number such that when the batteries are switched in circuit across the load, the cumulative voltage of the batteries exceeds a predetermined load voltage for a preselected period. The over-voltage that results from the switching in of extra cells across the load is down converted by a converter. The converter, a sensor for sensing the system discharge bus voltage, and a switch may be formed as a single unit using MOSFET technology. It is said that in such case a fail-safe contact switch might also be provided to parallel the MOSFET switch and be operated in the event of its failure.
U.S. Pat. No. 5,886,503, incorporated herein by reference, discloses an apparatus that selectively tests battery cells in a string of battery cells employed as a back-up power supply to a primary power source. The apparatus includes an isolation circuit for partially isolating the string of battery cells from the load bus while permitting current flow between the battery cells and the load bus in the event of a failure of the primary power supply. A switching network is also disclosed wherein at least two controlled switches are arranged to selectively form circuits for discharging one or more of the battery cells while the cells remain connected in the string. A logic circuit connected to the controlled switches determines and selects which of the battery cells will be discharged for testing.